Disclosed herein, in various embodiments, are stable, high performing conductive compositions suitable for printing, such as by inkjet, aerosol jet, gravure, and flexography printing. In particular, there is provided a conductive ink formulation comprising silver carboxylates which has optimal performance, such as, tunable viscosity for various printing technologies, substantially particle free composition to prevent inkjet nozzle clogging, lower annealing temperatures and improved stability.
Fabrication of electronic circuit elements using liquid deposition techniques is of profound interest as such techniques provide potentially low-cost alternatives to conventional mainstream amorphous silicon technologies for electronic applications such as thin film transistors (TFTs), light-emitting diodes (LEDs), RFID tags, photovoltaics, sensors, and the like. However the deposition and/or patterning of functional electrodes, pixel pads, and conductive traces, lines and tracks which meet the conductivity, processing, and cost requirements for practical applications have been a great challenge. Moreover, it is anticipated that printed electronics will involve multiple printing technologies which have different viscosity requirements, ranging from about a few cps for ink jetting to about 50,000 cps for screen printing. A conductive ink composition with tunable viscosity is therefore desired.
Solution processable or printable silver-based conducting materials have attracted attention for printed electronic applications. In this regard, fabrication of electronic circuit elements using silver nanoparticle ink compositions is known. Silver nanoparticle ink compositions are described in, for example, U.S. Pat. Nos. 8,361,350, 8,324,294, 8,298,314, 8,158,032, and 8,057,849, each incorporated herein by reference in its entirety.
The use of silver nanoparticle ink compositions may have some drawbacks, for example such as consistency in nanoparticle size and quality, which may negatively impact the formation of consistent and reliable conductive features and stability of the ink composition. Silver nanoparticle ink compositions can also be expensive to prepare and use.
Besides silver metal nanoparticles, another type of silver-based conductive material, silver-organic compounds, may offer several benefits such as lower-cost, better stability, and ease of preparation, when compared to silver nanoparticles with respect to use in conductive ink compositions. For example, silver carboxylates, such as silver neodecanoate, are low-cost, very stable silver salts that may be used to print conductive traces at a low processing temperature.
However, silver carboxylates such as silver neodecanoate have previously been synthesized in a two-step process, which involved expensive starting materials and required a large amount of water/methanol to remove sodium salt impurity. For example, silver carboxylate such as silver neodecanoate may be synthesized in a two-step process in which first, neodecanoic acid is reacted with an alkali base solution such as sodium hydroxide solution to form sodium neodecanoate in a water/alcohol mixture. The sodium neodecanoate then is reacted with silver nitrate to precipitate the silver neodecanoate salt, followed by washing with a large amount of water/methanol to remove the sodium salt. The drawbacks of this approach are clear: 1) expensive starting materials such as silver nitrate have to be used; and 2) a large amount of water/methanol is required to wash the product to remove the sodium salt impurity.
U.S. Pat. No. 4,723,024, incorporated herein by reference in its entirety, describes a method of preparing a silver salt of an organic acid comprising the steps of: (A) preparing a mixture of: (1) at least one organic carboxylic acid, (2) a hydrocarbon solvent, and (3) a mineral acid; and (B) adding a source of silver cation while maintaining the temperature of the resultant mixture at least at about 60° C. for a period of time sufficient to form the desired silver salt. The method described in this patent thus requires the use of a mineral acid and requires an elevated reaction temperature.
There remains a need for improved methods of making silver carboxylates, and conductive ink compositions comprised of silver carboxylates that have tunable viscosity for various printing methods and that are capable of forming high resolution traces having high conductivity, which methods are more economical than present methods.